Electrical circuits that protect rechargeable elements, such as rechargeable battery packs, are well known. However, such rechargeable elements, and in particular rechargeable lithium battery cells, can be dangerous if the operating voltage exceeds a safe limit.
For example, FIG. 1 shows a typical charging curve, i.e., the voltage across the battery vs. time, for a common lithium battery pack (e.g., used for a wireless telephone handset) allowed to keep charging beyond its maximum safe level. As labeled in FIG. 1, this curve may be divided into three general areas.
The first area is represented by the region where the voltage, V, is less han 4.5 volts. In this area, the battery charges at a safe level, with the temperature of the battery remaining below 60.degree. C. to 70.degree. C., and the pressure inside the battery remaining below 3 bars.
The second area is represented by the region where the voltage is between 4.5 volts and 5.3 volts. When charging is in this area, the battery begins to operate in a dangerous mode, with the temperature rising above 70.degree. C., and the pressure inside the battery rising to a range between 3 bars to 10 bars. Even at this slightly increased voltage level, the battery might even explode.
The third area is represented by the region where the voltage exceeds 5.3 volts. At this stage, it is too late to save the battery, which is subjected to internal degradation and may explode or combust. Notably, battery cells in a "fully-charged" state are more dangerous and susceptible to explosions than those in the discharged state.
In particular, in order to be sure that a lithium battery operates in its safe operating mode during a charging operation, at least one of the following three conditions must be met: 1) temperature &lt;60.degree. C., 2) pressure &lt;3 bars, or 3) voltage &lt;4.5 volts.
Towards this end, rechargeable lithium ion battery packs are conventionally provided with a "smart" electronic circuit in series with the batteries to provide protection against exposure to an excessive voltage or current. Such smart protection circuits may also guard against an undervoltage condition caused by overdischarge of the battery pack.
By way of example, a conventional "smart" protection circuit 21 for a rechargeable lithium ion battery pack is shown in FIG. 2. In particular, first and second MOSFET switches 20 and 22 are placed in series with one or more battery cells 24. The MOSFET switches 20 and 22 are switched ON or OFF by control circuitry 26, which monitors the voltage and current across the battery cell(s) 24. In normal operation, the MOSFET switches 20 and 22 are switched "ON" by the control circuitry 26 to allow current to pass through in either direction for charging or discharging of the battery cell(s) 24. However, if either the voltage or current across the battery cell(s) 24 exceeds a respective threshold level, the control circuitry 26 switches OFF the MOSFETs 20 and 22, thereby opening the circuit 21. The control circuitry 26 also monitors the voltage and current levels across a charging source 28 to determine when it is safe to switch back ON the respective MOSFETs 20 and 22.
As will be appreciated by those skilled in the art, the smart protection circuit 21 is relatively complex and expensive to implement with respect to the overall expense of a conventional battery pack. Further, the series resistance across the MOSFETs 20 and 22 is relatively high, thereby decreasing the efficiency of both the charging source 28 and the battery cells 24. Notably, both MOSFETs 20 and 22 are needed to prevent current from passing in either direction when the circuit is open,--i.e., by way of respective body diodes 23 and 25 biased in opposite directions--, which increases the complexity, cost and total in-series resistance of the protection circuit 21. Also, because the MOSFETS 20 and 22 are subject to failure if exposed to a sudden high voltage (or use of an improper high voltage charger), secondary protection of the battery cell(s) 24 is still needed, such as, e.g., a positive temperature coefficient ("PTC") resettable fuse employed in series with each cell.
By way of background information, devices exhibiting a positive temperature coefficient of resistance effect are well known and may be based on ceramic materials, e.g., barium titanate, or conductive polymer compositions. Such conductive polymer compositions comprise a polymeric component and, dispersed therein, a particulate conductive filler. At low temperatures, the composition has a relatively low resistivity. However, when the composition is exposed to a high temperature due, for example, to ohmic heating from a high current condition, the resistivity of the composition increases, or "switches," often by several orders of magnitude. The temperature at which this transition from low resistivity to high resistivity occurs is called the switching temperature, Ts. When the device cools back below its switching temperature Ts, it returns to a low resistivity state. Thus, when used as an in-series current limiter, a PTC device is referred to as being "resettable," in that it "trips" to high resistivity when heated to its switching temperature , Ts, thereby decreasing current flow through the circuit, and then automatically "resets" to low resistivity when it cools back below Ts, thereby restoring full current flow through the circuit after an overcurrent condition has subsided.
In this application, the term "PTC" is used to mean a composition which has an R14 value of at least 2.5 and/or an R100 value of at least 10, and it is preferred that the composition should have an R30 value of at least 6, where R14 is the ratio of the resistivities at the end and the beginning of a 14.degree. C. range, R100 is the ratio of the resistivities at the end and the beginning of a 100.degree. C. range, and R30 is the ratio of the resistivities at the end and the beginning of a 30.degree. C. range. Generally the compositions used in devices of the present inventions show increases in resistivity, which are much greater than those minimum values.
Suitable conductive polymer compositions are disclosed in U.S. Pat. Nos. 4,237,441 (van Konynenburg et al), 4,545,926 (Fouts et al), 4,724,417 (Au et al), 4,774,024 (Deep et al), 4,935,156 (van Konynenburg et al), 5,049,850 (Evans et al), 5,250,228 (Baigrie et al), 5,378,407 (Chandler et al), 5,451,919 (Chu et al), 5,582,770 (Chu et al), 5,701,285 (Chandler et al), and 5,747,147 (Wartenberg et al), and 6,130,597 (Toth et al). The disclosure of each of these patents is incorporated herein by reference for all that it discloses.
Referring to FIG. 3A, a crowbar type protection circuit 31 is also well known. In particular, a switch element 30 is placed in parallel across the battery cell(s) 24. The switch 30 is opened or closed by control circuitry 36, which monitors the voltage and current across the battery cell(s) 24. In normal operation, the switch 30 is left open. However, if either the voltage or current across the battery cell(s) 24 exceeds a respective threshold, the control circuitry 36 closes the switch 30, thereby shorting the circuit across the battery cell(s) 24.
FIG. 3B illustrates the current versus voltage curve 35 through the switch element 30, when it is closed. Notably, the current can quickly reach relatively high levels, depending on the characteristics and duration of a particular power surge. Towards this end, a first overcurrent element 32 may be provided between the switch element 30 and the charging element 28 to help protect the switch element 30 from continuous current from the charging element 28 (shown in FIG. 2). Similarly, a second overcurrent element 34 may be provided between the switch element 30 and the battery cell(s) 24, in order to protect the cell(s) 24. However, the combined in-series resistance of the overcurrent elements 32 and 34 is undesirable across the battery path.
FIG. 4A depicts an alternate overvoltage protection clamping circuit 41. In particular, a voltage clamping element 40, such as a zener diode, is used in place of the switch element 30 in parallel with the battery cell(s) 24. In an overvoltage condition, the clamping element 40 limits the voltage across the battery cell(s) 24.
FIG. 4B illustrates the current versus voltage curve 45 for the clamping circuit 41. As with the crowbar circuit 31, current through the clamp 40 can quickly reach relatively high levels, depending on the characteristics and duration of a particular voltage spike. Again, placement of current limiting elements (not shown in FIG. 4) can protect the clamp 40 and/or battery cell(s) 24 from excessive current. Notably, the clamping element 40 can have a relatively high current leakage, e.g., as in the case of a zener diode, causing the battery cell(s) 24 to lose their charge quickly over time.